More than eleven thousand patients require mechanical ventilatory support each day in the United States. As many of 41% of this vulnerable group of individuals experience difficulty during the process of weaning from mechanical ventilation. Weaning failure is attributed to a number of altered physiological states including cardiovascular instability. Despite the presumed importance of cardiovascular status to weaning, a comprehensive evaluations of cardiovascular function before, during and after weaning has not been reported. This study intends: 1) to perform a comprehensive evaluation of cardiovascular function prior to, during and following weaning from mechanical ventilation in a group of critically ill patients; and 2) to determine the independent predictive power of cardiovascular function to weaning failure. The specific aims of this study are: 1) to compare cardiovascular function (hemodynamic, autonomic, mechano-electrical and neurohormonal) before, during and after the transition from mechanical ventilation to independent spontaneous ventilation in a group of medical ICU patients after controlling for presence of cardiac disease; and 2) to determine whether weaning failure or success is associated with cardiovascular function after controlling for traditional predictors for weaning failure. Study will use a predictive correlational design and non-probably, convenience sample of 120 ventilated individuals. The data to be collected includes indices of 1) general physiological status; 2) pulmonary status; and 3) multiple aspects of cardiovascular status (hemodynamic, mechano-electrical, autonomic, neurohormonal). These data will be collected on baseline mechanical ventilation, during the weaning process and following extubation. A repeated measures analysis of variance will determine whether the measures of cardiovascular function differ across time (before, during and after weaning). A logistic regression will be used to determine predictors of weaning success or failure. Appropriate identification of those individuals at risk for failure to wean from mechanical ventilation due to cardiovascular dysfunction will provide the clinician with an opportunity to optimize cardiovascular function and increase the likelihood of weaning success.